1. Field of the Invention
The invention relates to a digital signal processing system and, more particularly, to a digital signal processing system used with a reproduction apparatus having a dropout compensation function for reproducing audio information signals recorded on a recording medium in the form of digital data words together with television synchronizing signals.
2. Description of the Prior Art
A digital audio system for recording and reproducing digital audio information in a format like that of a video signal using an existing home-use video tape recorder or a video disk apparatus, has attracted attention recently. The recording/reproducing system, generally called a PCM (pulse code modulation) audio recording/reproducing system, can remarkably improve noise, distortion, wow-flutter and the like. In the system, however, due to scratches of a magnetic tape or a disk or dirts thereon the signal dropouts often occur during a signal reproduction. In preparation for the dropout, the recording/reproducing apparatus such as a video tape recorder is usually provided with a dropout compensation circuit which interpolates a dropped out signal with a signal before one horizontal scanning line. The dropout compensation is based on the fact that a video signal such as a television signal has an extremely high correlation between signals on adjacent horizontal scanning lines. Unlike the video signal, however, an audio signal conversely is extremely low in such a correlation. If the technique of the dropout compensation for video signal is applied to the audio signal dropout compensation, a dropped out audio signal is interpolated by a signal which is not correlated to the dropped out signal, leading to signal quality deterioration such as increase in noises.
The PCM recording/reproducing apparatus has an error correction/revision circuit arranged to detect whether a digital word reproduced is in error or not by an error detecting code, and correct the erroneous word when an error is detected, or revise or conceal the erroneous word by an average value of correct words which are adjacent to the erroneous word and have a high correlation therebetween if the correction is impossible. In the PCM recording/repeoducing system using the above-mentioned recording/reproducing apparatus having the dropout-compensation circuit, when the signal dropout takes place, the error detection is impossible, since no error is contained in the data word interpolated with the data word before one horizontal line period. Therefore, the error correction/revision circuit cannot revise the interpolated data word as an erroneous data word. When a dropout-compensated data word is present, there is a great possibility that the error correction/revision circuit erroneously corrects other erroneous data. The dropout compensation function of the video tape recorder or the video disk apparatus is almost ineffective for the PCM audio system. For the PCM audio recording/reproducing system using the apparatus with such a dropout compensation function, it is desirable to be able to detect the dropout compensation and to cause the error correction/revision circuit to properly correct or revise erroneous data words.